Printing presses, especially rotogravure ones, usually comprise a plurality of different printing units or elements, each of which is adapted to print only one color on the support or web. As a consequence, the composition of a colored image needs a sequence of several printing units, one for each color.
The ink or the lacquer used to print is usually diluted with organic solvents such as ethyl acetate, ethyl alcohol and others. Once the ink is printed on the support, the solvent must be removed, e.g. by evaporation, and to this aim the printed support is dried using a hot air flow, generated inside a tunnel or oven through which the printed support is led. The warm air used to dry the solvent on the support may be partly reused in order not to waste too much heat.
The volume concentration of solvent vapors in the air that is reused in the drying process must be kept under strict control, because such vapors can even explode if their concentration (cubic meters of solvent vapor for each cubic meter of air) is between two values known as “Lower Explosion Limit” (L.E.L.) and “Upper Explosion Limit” (U.E.L.).
The amount of air that is reused in the drying process varies for each printing unit depending on the amount of ink and solvent that each printing unit leaves on the web. For instance, a printing unit that prints only few signs in the final image needs a smaller amount of ink than another printing unit that prints, for instance, the background of the image with a high covering of the printing surface. In this way, it is possible to reuse a bigger amount of air and accordingly waste a smaller amount of heat, because even the small amount of air that goes out of the printing unit is sufficient to maintain a low concentration of solvent vapors in the drying tunnel taking away the sufficient amount of solvent.
In many printing presses the amount of air that is reused is fixed during the design and installation of the printing press in order not to exceed 50% of the L.E.L. concentration under any operational condition.
In such presses the recirculation ratio is fixed by the manufacturer in order to guarantee an amount of fresh air coming into the drying oven of each printing unit sufficient to maintain a solvent concentration below 50% of L.E.L. concentration under the worst operational condition.
In real operational conditions, only few printing units of the press work, as said before, with a high ink covering of the web. In the other printing units, working with low ink covering, the drying process takes place with an amount of fresh air higher than necessary, since a very small amount of solvent has to be removed from the web by evaporation. In these printing units the concentration of solvent in the drying air is very low, e.g. 10-15% of L.E.L.
Since it is not established which printing units will provide a high ink covering, the recirculation ratio is based on the highest ink covering that the printing unit can print and equally fixed by the manufacturer for each printing unit.
As a result of the fixed recirculation ratio, to prevent the danger of explosions, in some printing units an amount of fresh air higher than necessary is used. A higher fresh air intake means higher heat necessary for the drying process, larger exhaust air ducts and larger and less efficient exhaust air treatment installations.
It is possible to have more sophisticated measuring systems that can manage the amount of air reused by each printing unit. A system that controls the concentration in more than one point for each printing unit is safer and makes it possible to have the press working by adjusting the recirculation in order to maintain a solvent concentration up to 50% of the L.E.L. concentration even with small quantities of solvent to be evaporated from the web.
With a recirculation ratio adjustment it is possible to set different recirculation ratios on different printing units. As a result, a smaller amount of fresh air is necessary in the drying process of the printing units working with low ink covering meaning lower exhaust air flow from the whole press, and lower heat necessary for the drying process.
The main difference with the fixed recirculation is that for each printing unit is required only the necessary fresh air depending on the real amount of solvent that must be removed from the web in that printing unit.
It is known that the installation constituting the printing machine is not closed, because the printed support needs to travel through the drying tunnel, which must be kept in a depressurized condition in order to prevent the air containing solvent inside the drying tunnel from exiting toward the ambient.
The depressurized condition also causes an amount of ambient air to penetrate into the drying tunnel, at those locations where the printed support enters or exits from the drying tunnel. This air, known as “false air”, dilutes the air inside the drying tunnel and must be extracted out of the drying tunnel for maintaining it at a negative pressure with respect to the ambient.